The present invention relates to heat sealing thermoplastic materials together, and in particular to a method of heat sealing a thermoplastic lid to a thermoplastic drum designed for the disposal of hazardous and nuclear wastes and to an article of manufacture comprising a sealed container.
A search of the prior art failed to uncover any prior art references which disclose the method or article of manufacture of the present invention. A number of patents were uncovered which disclose methods or devices for heat sealing or bonding methods in general.
U.S. Pat. No. 4,349,219 discloses a welding muff of thermoplastic material which forms a sleeve around two pipe parts that are to be joined. A resistance wire within the muff welds the interior surface of the muff to the exterior surfaces of the pipe parts.
U.S. Pat. No. 4,029,837 discloses a composite plastic-metallic bonding means and method comprising a metallic mesh interposed between first and second layers which are inserted between first and second bodies. The metal mesh is heated by exposing it to an alternating magnetic field. The heated mesh melts layers and forms a seal between the first and second bodies.
U.S. Pat. No. 2,961,363 discloses a method for coupling thermoplastic components comprising a hollow ring of an exothermic autocombustible substance which is placed between the two thermoplastic components to be coupled. Ignition of the exothermic autocombustible substance heats the thermoplastic components and welds them together.
U.S. Pat. No. 4,106,969 discloses a method of joining a thermoplastic material to an inorganic material comprising heating a helical coil and partially embedding it in the thermoplastic material and then gluing the inorganic material to the thermoplastic material using the helical coil as an anchor for the adhesive.
Because of inherent structural limitations or flammability problems, the above methods and devices are not suitable for welding thermoplastic materials or for sealing and storing hazardous waste materials.
In addition to the aforementioned prior art methods and devices, the following patents were cited by the Examiner in the aforesaid copending application, Ser. No. 638,556, now U.S. Pat. No. 4,586,624.
______________________________________ U.S. Pat. No. U.S. Pat. No. ______________________________________ 2,274,839 3,305,668 2,741,402 3,348,640 2,744,655 3,852,570 2,795,348 4,110,145 2,802,593 4,436,488 3,079,458 4,470,193 3,084,242 4,493,985 3,095,112 ______________________________________
The above-listed patents cited by the Examiner do not disclose the heat sealing method and article of manufacture of the present invention.
In recent times, the disposal of hazardous waste material has become a significant problem. Nearly 270 million tons of hazardous waste material is generated each year. Presently, the waste material is typically disposed of by placing it in either a metallic or plastic container. The metallic containers are unsuitable because they are subject to corrosion and leakage when exposed to the elements for a period of time.
The disposal of low level radioactive wastes is also a public and industry concern. Government regulations require disposal containers to have a 300-year design life and must have a positive seal of equal integrity.
To avoid the corrosion problems associated with metal containers, hazardous waste materials, such as solid hazardous wastes in bulk or particulate form, dewatered hazardous sludges, or hazardous wastes that have been stabilized through solidification/cementation processes, are frequently stored in plastic containers. The presently used plastic containers are closed by a conventional metal clamp holding the lid to the drum. It has been suggested that waste material originally placed in a metal container may then be placed in a plastic container.
It has also been suggested to heat seal plastic drums by suspending a heating plate between the plastic drum and a plastic lid that is suspended above the heating plate. When the top of the drum and the bottom of the lid reach a predetermined temperature, the plate is removed and the lid is pressed onto the drum to complete the seal.
Both methods for sealing the plastic containers are disadvantageous. The metal clamp is prone to corrosion or rusting and the heating plate method is awkward to use and requires expensive capital equipment.